The invention relates to a method and an apparatus for focusing a beam of light, in particular a laser beam, on an object by observation of the beam image on the object.
The processing of workpieces by means of laser beams is becoming increasingly important. One of the most essential problems in this context involves focusing the laser beam on the one hand onto the surface to be worked in an optimum manner and, on the other hand, to constantly maintain this focusing and to suitably adjust it.
The essential focusing requirements are rapidity, high sensitivity and maximum precision in one respect, and long term stability in the other.
It is accordingly an object of the invention to provide a method and apparatus satisfying the foregoing requirements.
The method and the apparatus according to the present invention to achieve this objective are described hereinafter in detail and set forth in the independent claims. Particularly appropriate and advantageous configurations and further details are set forth in the dependent claims.
Briefly, the invention is based on using a combination of two different focusing techniques, both of which detect and evaluate the light reflected by the beam spot on the object impacted by the light or laser beam, but in keeping with entirely different criteria. The two measuring techniques are suitably selected so that they complement each other and their respective inherent errors and shortcomings are mutually compensated for or become negligible.
For the continuous focus adjustment including follow-up regulation itself, one of the techniques described in DE No. 20 34 341, U.S. Pat. No. 3,689,159, DE No. 24 53 364, FR No. 94 871 or U.S. Pat. No. 3,614,456, may be used, wherein the beam spot is reproduced by means of an imaging objective on two or more diaphragms placed at different distances from the image plane or oscillating with respect to said image plane, and the light not shaded by the diaphragms is evaluated. The embodiments of these methods according to the invention, to be explained hereinbelow, are particularly appropriate and advantageous.
A technique utilizing the phenomenon of the so-called "speckling" (granulation of the light reflected by the beam spot) is particularly suitable for calibration; it will be described more fully hereinafter. The physical phenomenon of "speckling" is described for example in the following references:
R. K. Erf: "Speckle Metrology", Academic Press (1978) PA0 J. C. Dainty: "Laser Speckle and Related Phenomena", Springer Verlag (1975) PA0 K. A. Stetson: "A Review of Speckle Photography and Interferometry", Opt. Eng. 14, 482 (1975) PA0 J. C. Dainty: "The Statistics Of Speckle Patterns", Progress In Optics, Vol. 14, pp. 3-46 (1976).
The principal applications of "speckle" methods heretofore have been the characterization of surfaces and the measurements and displacements of test objects. In the context of focusing laser beams, however, speckle methods have not been mentioned heretofore. The principal advantage of focusing by the "speckle" method consists of the fact that the method is entirely insensitive with respect to the adjustment of the optical components. The focusing method therefore represents an ideal supplement of the first mentioned focusing methods, which are highly critical in this respect.